Isocyanates, which are widely used as intermediates of polyurethane, adhesives, herbicides and pesticides, have been synthesized from amines and phosgene. However, since the method uses highly toxic phosgene, it has problems such as phosgene leak, equipment corrosion and the production of a lot of HCl causing pollution as a by-product. Therefore, there has been a need to develop a new eco-friendly method which does not use phosgene. Also, in view of the tendency that regulations on toxic substances are globally strengthened, research and development on green technology to synthesize isocyanates by non-phosgene process can be quite attractive in the chemical industry.
The isocyanates can be largely divided into aliphatic isocyanates and aromatic isocyanates, both of which are used as useful industrial raw materials. For example, polyurethane, which is prepared using aromatic 4,4-diphenylmethane diisocyanate (MDI) as a raw material, is widely used for microporous elastomer, thermoplastic elastomer, casting elastomer, artificial leather, synthetic leather, adhesive, coating, sealant, etc. Also, polyurethane form plastic, which is prepared using aromatic diisocyanates, 2,4- and 2,6-toluene diisocyanate (TDI), is used for elastomer milled with polyurethane elastomer, coating, adhesive, waterproof material, detergent, thickener, antioxidant, etc.
However, the polyurethane prepared from the aromatic diisocyanates has the problems of being yellowed by light after being exposed to the outside, thereby increasing the use of polyurethane prepared from non-yellowed aliphatic diisocyanates. For example, 1,6-hexane diisocyanate (HDI) is used for surface treatment paint and advanced coating for mainly car coating, surface treatment paint for anti-corrosion coating of airplanes, paint for enamel-insulated wire, paint for wood furniture, surface treatment paint for trains, polyurethane glue material having good light stability, rocket propellant additive, etc. In addition, 3-isocyanatomethyl-3,5,5-trimethylcyclohexylisocyanate(isophorone diisocyanate: IPDI) is widely used for aliphatic polyisocyanates such as polymers, trimers and adducts, waterproof polyurethane coating, and various plastic products such as polyurethane leather, etc. Moreover, polyurethane prepared using 4,4-dicyclohexylmethane diisocyanate (HMDI) as a raw material is widely used for foam plastic, elastomer, coating, adhesive, anti-corrosion coating, paint for wood furniture, etc.
Among the eco-friendly methods to prepare isocyanates by non-phosgene process which have been known so far, the most economical and studied method is to prepare the isocyanates by reacting amines with dialkyl carbonate or urea/alcohol to synthesize carbamates, followed by pyrolyzing them. However, since a large amount of polymer by-products generated during the pyrolysis of the carbamates lower the isocyanate yields and block the flow of liquid to stop the equipment, various studies have been tried to solve the problems.
U.S. Pat. No. 3,962,302 discloses a method to synthesize TDI by pyrolyzing toluene diethylcarbamate, which flows nitrogen into the reactor in the presence of cetane solvent at 250° C. to obtain TDI with 83.4% yield. However, since the method uses nitrogen, it cannot be applied to practical process. U.S. Pat. No. 4,294,774 teaches a method to prepare MDI with 46% yield using N,N-dimethylaniline as a solvent and catalyst. Also, U.S. Pat. No. 4,349,484 discloses a method to prepare MDI with 76.5% yield by decomposing diphenylmethane dimethylurethane at 310° C. using zinc pieces as a catalyst in the presence of dicyclobenzene solvent. However, the above methods have the problems of low MDI yields, degradation and loss of the solvents due to vacuum and high temperature, and the generation of a large amount of high-boiling materials. In addition, DE Pat. No. 1,022,222, U.S. Pat. No. 3,919,279, DE Pat. No. 2,635,490, U.S. Pat. Nos. 4,873,365 and 4,386,033 disclose methods to prepare IPDI, HMDI or HDI by decomposing dicarbamates in the presence of a basic, acidic or organic metal compound catalyst and a suitable solvent, but they cannot avoid the formation of high-boiling by-products.
To reduce the formation of the high-boiling by-products, U.S. Pat. No. 4,388,246 discloses a method to prepare diisocyanates by pyrolyzing dicarbamates using hydrogen chloride, chlorinated organic acid, alkyl ester having alkylation function, or inorganic acid salt of alkyl ester as a stabilizer and organic tin chloride compound as a catalyst. Despite using the catalyst and stabilizer, the formation of the high-boiling by-products was not significantly reduced, and the diisocyanate yields were not high enough.
For these reasons, methods which use heavy metal such as Co, Mn, Fe and Ni, or metal compounds thereof as a catalyst were proposed, but they also had the problems that high-boiling by-products were accumulated in the reactor since high-boiling by-products were generated, thereby reducing the catalyst activity.